In general, electric motors used in cars and the like are equipped with a circuit breaker using bimetal or the like in order to prevent burnout of armatures caused by continuous use in an overloaded state. In addition, it is necessary for a system having a motor control circuit to prevent breakdown of not only armatures but also switching elements such as FETs. This kind of system adopts a method of detecting an electric current to protect elements, in many cases. In this case, a current sensor or the like which detects the amount of an electric current, depending on changes in peripheral magnetic flux caused by changes in current amount, is used to detect the electric current, and a current detection circuit in a controller detects an over current.
For another system of a type which controls driving of a motor by obtaining a pulse output according to rotation of the motor, there has been a known method of determining a state of the motor depending on whether a motor pulse is detected or not. In a motor of this kind, the motor pulse is constantly detected during rotation. If no pulse can be detected while a voltage is applied, the motor can be determined as having been locked. Hence, when the motor comes into a pulse-undetected state, the motor output is stopped to avoid continuous use in an overloaded state. Thus, the motor is prevented from being damaged.
In the method of using a circuit breaker or a current sensor, however, components of the breaker or sensor have to be included in circuits and hence cause a problem that the system becomes expensive. If a current detection circuit is provided, the whole size of the controller becomes large accordingly. In the case of a motor which has only a small capacity space, like a motor for a car, there are limitations concerning layout. Therefore, another problem arises in that no space can be maintained to provide the controller with the current detection circuit, depending on the installation location.
On the other side, the other method of determining an overload depending on the motor pulse does not use a current sensor or the like. Hence, the system can be constructed at low cost. In this method, however, if an overloaded state continues for a long time period without causing the motor to be locked, the motor continues operating even at a low speed, and so, a motor pulse can be detected. Although the motor is in an overloaded state, the overloaded state cannot be detected adequately. As a consequent, a problem arises in that the motor is damaged.
The present invention has an object of providing a motor control method and a motor controller capable of adequately knowing an overloaded state of the motor.